More mesh levelling issues

Quotecheeseandham
This is my last mesh with lokbuild stuck on my aluminium plate.

If you're wondering about the flatness of my aluminium plate, it is Aluminium ACP 5080 6mm, with a base tolerance is 0.1mm across a length of 3.3m x width 2.05m.
Then it was cut to 203mm x 203mm and ground flat to a tolerance of 0.4 microns.

It's pretty flat.

Quotedc42

That pattern indicates that there is a problem with your X axis. At low X values, the trigger height is depending on the direction in which the X axis last moved.

dc42,

I understand you are a busy man (and a personal thank you for all your work on the Duet!), but do you have any further explanation or advice - as a fellow Ormerod owner?

I can't think where this "X axis issue" could be...

This is my latest 4 point bed comp. after getting rid of my Y-axis twist last night. It's is now flat, and measuring with the nozzle shows that this seems to be "true" (for a given value of true)

The x carriage parts are solid, there is no movement except for left and right.

the mini diff. sensor is mounted and has no wiggle. While writing this away from my printer, I think that the sensor may not be exactly,truly perpendicular to the plate (although very close) - could this cause?

as mentioned, the plate is 6mm Al with a high tolerance, using lokbuild which is not transparent. (I can understand the lokbuild has lettering on it, but the problem is not limited to that area.)

Of course, I could be wrong about any of the above points, but that is how it appears to me.
I guess I don't really have a problem, in the sense that the plate is flat , the printing appears to be fine (a small extrusion issue occasionally, but I've been changing filaments a lot recently, so I think my temps aren't high enough sometimes) and so I guess the mesh isn't "necessary".

But you know what it's like, you have to get to the bottom of these things!

1. The wave. This suggests that either the height or the angle of the print head varies with X position.

2. Ridges and valleys in the X direction on alternate rows of probe points. This indicates that the height or angle of the print carriage depends a little on whether you approach the point in the +X or -X direction.

3. Isolated spikes and depressions, which were not present (or very much smaller) on your previous height map. These are most likely caused by the trigger height of the IR sensor not being consistent. Has the bed surface changed since you generated the previous height map?

My Ormerod is currently decommissioned due to lack of bench space, but I'll try to extract a height map from the SD card and post it tomorrow.

Quotecheeseandham
I'll take a close look at the cabling and the PTFE tubing, and see if it is doing something.

The PTFE tube _is_ doing something, mostly with the acrylic rib, less with the aluminium one, but still via the springiness of the plastic parts between x-runner- bearing and metal part of hotend
[forums.reprap.org]

Unfortunately a well known 3D Printing part supplier thinks it is fair play to offer next day delivery on orders placed in good time on a Thusrday amd not deliver until a Monday. I'll be having a real close look at this when I get the coupler.

Quotedc42
My Ormerod is currently decommissioned due to lack of bench space, but I'll try to extract a height map from the SD card and post it tomorrow.

Fair enough, approaching a similar problem myself. Thanks all the same, it'll be great to see what we can reasonably expect from the modulated sensor. Aluminium bed support or mdf on yours? Ormerod 1 or 2!

2. That's an interesting one, as this map was using "M558 P1 H5 F120 T6000 A5 S0.03" so is doing multiple probes. I would've assumed (obviously wrongly) that since it's just going up and down to verify the height that there would be no approach direction bias.

3. The bed surface has not changed, however as the Lokbuild does have lettering on it, since the previous height maphas less resolution on (80 points / S20 vs 300+ points / S10) prehaps the those spikes are due to the lettering?

Quoteormerod168
The PTFE tube _is_ doing something, mostly with the acrylic rib, less with the aluminium one, but still via the springiness of the plastic parts between x-runner- bearing and metal part of hotend
[forums.reprap.org]

Erik

Thank you Erik. So to mitigate this in your opinion what should be done?

One idea is to increase the size of the bowden as much as reasonably possible, but I'll be very interested to hear anything else.

The pre-load is already pretty high in my estimation. It was a lot more than my previous Ormerod 2 , and I increased the current from 800 to 1000 to assist it moving well.
The only play I can find of any kind is that the hotend can move up and down very slightly as the 9mm bearing
which to my mind is standard in Ormerods, (I thought one of the advantages of the cantilever cartesian design was that the cables put a downward force on the hotend, but I may be mistaken there)
on the but I can't replace the 9mm bearing with a 10mm bearing as the part isn't big enough (I guess I'd have to get the stl and make the hole a little larger)

increased the current to 1200 - no effect

loosened the wires and held them as well as I could to reduce any force on the hotend during a mesh scan - no effect

replaced the sensor mount for the IR (just in case and I was going to do it anyway as it was PLA and the rest of my build is PETG) - no effect

loosened the belt - no effect

(re)tightened the belt - no effect

placed a 150g weight on the hotend while scanning to transmit an extra downward force (whilst cables were loose and bowden unhooked) - no effect

However in thinking about the preload, I tried to measure it (with scales and a rod , lol, unsuccessful) and then with my eyes closed, run the X carriage by hand up and down the linear bearing.

It does feel (which is obviously subjective) that the carriage is harder to move where there are hills, and easier to move where there are valleys.
When I did the rebuild recently, I was very careful about choosing a linear bearing and rod that felt like it was running up and down smoothly (as I have several)

So, I will investigate further (got to leave now) but the question is: what could be causing that?

On my recent look through the machines and shops it's become apparent that the ball bearing style bearing has fallen out of favour and dry maintenance free bearings are more popular. Igus Drylin I think it was called. Not sure if that is your issue but I think the bearings do run in a loop. One thing to be said for your deviation is it's a nice looking sine wave! I wonder if the wavelength is close to the distance the bearing has to slide to run the bearings through a full loop?

Quotedc42
I've seen elsewhere in these forums that those in the know advise against using linear bearings. Linear rails are recommended instead.

I entirely agree, and I'm starting to build a CoreXY with linear rails (Based heavily on the Railcore 2 ZLT and it's components, just a few changes).
However I'm not good enough with CAD to redesign the Ormerod 2 to work with linear rails

Perhaps I'm coming to an issue with the machines design? I don't think mesh compensation wasn't around when RepRapPro stopped business.

Wes, have you fitted a dc42 mini-diff sensor yet? Do you get anything like what I am seeing with a mesh compensation with your Ormerod's?

1. The wave. This suggests that either the height or the angle of the print head varies with X position.

2. Ridges and valleys in the X direction on alternate rows of probe points. This indicates that the height or angle of the print carriage depends a little on whether you approach the point in the +X or -X direction.

I have a busy couple of days ahead, so I'm going to meditate on these two points for a while

Quoteormerod168
The PTFE tube _is_ doing something, mostly with the acrylic rib, less with the aluminium one, but still via the springiness of the plastic parts between x-runner- bearing and metal part of hotend
[forums.reprap.org]

Erik

Thank you Erik. So to mitigate this in your opinion what should be done?

One idea is to increase the size of the bowden as much as reasonably possible, but I'll be very interested to hear anything else.

I don't like the idea of a longer bowden for retraction reasons and the cabling would still be a problem esp. on the ormerod2 - a much more robust x-runner in aluminium would solve those problems once and for all

Last time I rebuild the X-arm I took out all other variables, checked the smooth rod in a lathe, mounted the X-rib of the X-arm in a wise, milled a grove in the X-axis-plate and imbedded a hacksaw blade for the bearing to run on